Ram air turbine (RAT) driven auxiliary power systems are commonly used in the aircraft industry to provide power for "add-on" equipment such as pod mounted aerial refueling equipment or weapons systems. In addition, most modern commercial and military aircraft carry a RAT driven auxiliary power unit to provide a source of emergency hydraulic or electrical power for critical flight controls and landing gear, should the aircraft experience a total loss of power from all engines.
In a typical RAT driven auxiliary power unit, the actual ram air turbine, which looks very much like an aircraft propeller, is mounted on the outboard end of a support strut attached to the aircraft and extending into the surrounding airstream. The RAT is coupled by a drive mechanism to drive one or more power converting devices, such as a hydraulic pump or an electrical generator.
During flight, the airstream generated by the forward motion of the aircraft causes the RAT to rotate. The RAT drives the pump or generator to produce hydraulic or electrical power which is delivered through hydraulic lines or electrical cables to the appropriate aircraft system.
The power converting devices are typically located either within the aircraft fuselage, or mounted on the strut.
Where the power converting devices are located within the fuselage, the drive mechanism must often follow a torturous path through the strut in order to connect the RAT to the power converting device. A complex drive mechanism is usually required in such instances. This is particularly true in those instances where the strut must pivot about an axis located within the aircraft, such that the auxiliary power unit may be pivoted between a stowed position within the aircraft and a deployed position outside the aircraft.
In general, where the power converting device is located within the fuselage, the common practice has been to fabricate a strut with a cylindrical cross section and to route the drive mechanism through the inside of the cylinder. U.S. Pat. Nos. 4,742,976; 4,991,796; and 5,174,719, which are assigned to the assignee of the present invention, are illustrative of this approach. While this approach does provide certain benefits, such as protecting the drive mechanism from the elements and inherent resistance of the strut to torsional loading, there are also significant penalties incurred due to the difficulty involved in fabricating a hollow strut. Furthermore, assembly and repair of the drive system within the enclosed strut often requires that special provisions be made to allow access, or that the strut be assembled from multiple parts.
Mounting the power converting device on the outboard end of the strut greatly reduces the problem of connecting the drive mechanism to the RAT. Even with this approach, however, hydraulic lines or electrical cables must still be routed through the strut in order to transmit the converted power from the power converting device to the aircraft. If cylindrical struts, or struts having other internal passages are used to route the lines or cables through the interior of the strut, the difficulties involved in fabricating a hollow strut are not avoided. Also, unless additional access features are provided, it is difficult if not impossible to include convenient intermediate connections within the strut to facilitate connection of the hydraulic lines or cables.
Hollow, cylindrical shaped struts are inherently difficult to fabricate. This is particularly true where the strut must also incorporate complex features such as mounting bosses, bolt flanges or access covers. Casting or molding are typically the manufacturing methods of choice for economical fabrication of such structures. In order to cast or mold such shapes, however, complex multi-part tooling and fungible cores are typically required. Alternatively, the strut is fabricated in several parts which are bolted or welded together to produce the strut. Secondary machining operations are typically required. The need for complex tooling, fungible cores, multi-part construction, and secondary machining or joining operations drives fabrication costs up.
This is particularly true for structures fabricated from fiber reinforced composite materials, wherein features such as bolted joints are highly undesirable. Bolted joints are difficult to accommodate in a composite structure without severely impacting structural integrity. As a result, the complex, hollow cylindrical, shape utilized for prior RAT support struts has largely precluded the use of composite materials. Accordingly, the significant advantages offered through the use of composite materials, such as reduction in weight, superior corrosion resistance, high specific stiffness, and virtually unlimited fatigue life, could not be utilized to advantage in prior RAT support struts.
What is needed then, is a new RAT support strut configuration which is readily manufacturable at low cost from a variety of metallic and non-metallic materials including fiber reinforced composites. Ideally, such a new strut would be readily molded in a single step as a one-piece unitary structure.